Joe's definition- leaving out one word- becomes "a song in which something happens." Traditional implies age, unknown author, change, etc., which means a person can't sit down today, write a song, and call it a ballad.
Leave out the traditional, and the definition might be workable.
Dictionaries, however, allow broader definitions.
Sandy McLean perhaps asks too much. "You Ain't Nothin' but a Hound Dog" certainly imparts a message. "Cryin' all the time" and other lines show something is happening. Is it or ain't it a ballad?

The Oxford Dictionary lists this definition first:
1. A song intended as the accompaniment to a dance; the tune to which the song is sung. The original meaning, but now obsolete in their view.
2. A light simple song of any kind, now specifically a sentimental or romantic composition....the musical accompaniment being strictly subordinate to the air.
3. A popular song; often one specifically celebrating or....attacking persons or institutions....Often printed as a broadsheet. Obsolete [Not to many of us- only to the general public is this meaning obsolete].
4. A proverbial saying. Obsolete.
5. A simple spirited poem in short stanzas, originally a ballad in sense 3 in which some popular story is graphically narrated. ("This sense is essentially modern; with Milton, Addison, Johnson, the idea of song was present").
The word has been used as a verb, but now more or less obsolete ("to ballad").

From this, we see that the sense of a story or a song with a moral point (Sandy McLean, above) is a modern one. The word started out meaning a song accompanying a dance, so there has been a shift in meaning.

Perhaps Joe's definition (leaving out the word traditional) is as good as any. I see so much overlap, however, with anything with words, that I am happy with simple "song."